1. Field of the Invention
The present invention relates to a motor controller, and in particular to a motor controller which is used as a drive source of various machines such as machine tools, industrial equipments, and robots, and in which feedback control of a drive current for driving a motor is conducted.
2. Description of the Prior Art
In a motor controller which detects a current flowing through a motor and which utilizes the detected current for current control of the motor, the detected motor current is converted into a unit which can be readily utilized for control, such as voltage, with a certain conversion ratio, and then utilized. Furthermore, in the motor controller, there is provided a function of detecting an overcurrent and interrupting the motor current, in order to protect a permanent magnet motor from destruction caused by demagnetization in the case where the control is disabled or in the case where switching power elements are subjected to short mode destruction.
FIG. 2 is a block diagram of a principal part of a motor controller for effecting feedback control of this motor current. Reference numeral 10 denotes an inverter circuit for generating AC power of an arbitrary frequency from a DC power source E. In this inverse circuit 10, diodes D1 to D6 are connected parallel to switching elements T1 to T6 formed of power elements such as IGBTs, in the reverse direction, respectively, forming 6 sets of parallel circuit. Further, these 6 sets of parallel circuits (T1-D1; T2-D2; . . . T6-D6) are arranged 3 sets of series connections each composed of two parallel circuits. And these 3 sets of series connections are connected in parallel to the DC power source. Moreover, a connection point between 2 sets of parallel circuits which constitute a set of series connection is connected to one of U, V and W phases of a motor.
A current flowing through each phase winding is detected by a current detector 20, and converted to a unit which is easy to be used for control of voltage or the like. This converted voltage is fed back to current control means 40. On the basis of a current command and the detected current value thus fed back, the current control means 40 effects current feedback control, and outputs a PWM generation signal to PWM signal generation means 60. On the basis of the PWM generation signal, the PWM signal generation means generates a PWM signal which turns on/off the switching elements T1 to T6 of the inverter circuit 10.
Furthermore, an overcurrent detection circuit 50 is provided to the motor controller. If a signal obtained by converting a current detected by the current detection means 20 to a voltage exceeds a preset level, then the overcurrent detection circuit 50 outputs a signal to the PWM signal generation circuit 60 and thereby inhibits a PWM signal output from the PWM signal generation circuit 60.
As described above, the current detection means 20 converts the detected current value to an easily usable voltage and outputs the voltage. The conversion ratio between the current and voltage in the current detection means is fixed in the prior art. On the other hand, it is desirable that the signal converted from current into voltage has a dynamic range wide as far as possible in order to stabilize the motor control.
For example, in a motor which is at least 50 A in motor demagnetization limit current, the current detection means 20 in the controller converts a commanded maximum current 40 A to a voltage 4 V, and an overcurrent protection voltage (i.e., a voltage at which output of the PWM signal is inhibited and the switching elements are turned off) is set to 5 V. In FIG. 2, this example is shown, and a full range for current control is set to 4 V and an overcurrent detection level is set to 5 V.
On the other hand, in a motor which is at least 100 A in motor demagnetization limit current, an commanded maximum current 80 A is converted to a voltage 4 V, and an overcurrent protection voltage is set to 5 V (5 V.times.80 A/4 V=100 A).
In the prior art motor controller, the conversion ratio to voltage or like in the current detection means is fixed. In addition, the conversion ratio is determined on the basis of conditions of the motor maximum current and the motor demagnetization limit current. Accordingly, there occur restrictions in the combination of the motor controller and the motor.
Especially, in the case of a multi-shaft controller in which one motor controller drives and controls a plurality of motors, there is a problem of an increased number of necessary machine kinds (current detection means differing in conversion ratio).